The present invention relates to an active matrix liquid crystal display which has thin film transistors or thin film diodes used as switching elements. More particularly, it relates to a liquid crystal display which excels in speed of response and viewing angle and which can display images in gray scale.
Liquid crystal displays (LCDs) is very advantageous over other types of displays since they are smaller and consumes less power. Actually, LCDs are used in personal computers and some other apparatuses. It is expected that they will be used as flat-panel displays in the future, in ever increasing numbers. Furthermore, new types of LCDs are being developed.
Nematic and Super Twisted Nematic Liquid Crystal
LCDs are classified into various types on the basis of the method of driving the liquid crystal. Of these types, the TN (Twisted Nematic) type and the STN (Super Twisted Nematic) type are widely employed at present. The TN-type LCD has TFTs (Thin Film Transistors), which drive nematic liquid crystal. In the STN-type LCD, nematic liquid crystal with an increased twist angle is driven by external means. Full-color, 10-inch to 20-inch LCDs of both types have been developed and are now commercially available for use in information-system terminals.
The TN-type LCD has acquired operating abilities which are fairly satisfactory for limited use, for example, in word processors and tabulation-calculation. The STN-type LCD has a response speed, but not so high that the LCD may be used in word processors or tabulation-calculation. Its viewing angle is too small. A retardation film, for example, is used to increase the viewing angle, but no sufficient angle has yet to be obtained. Thus, the STN-type LCD is not satisfactory in terms of response speed or viewing angle.
The TN-type LCD exhibits a fairly high response speed. When its screen size is increased, the TN-type LCD may no longer have a sufficient response speed because of the unnatural image of moving picture. The TN-type LCD has indeed a wider viewing angle than the STN-type LCD, but its viewing angle is not adequate for displaying full-color images. Because of its inadequate viewing angle, the TN-type LCD finds but limited uses.
As indicated above, it is much desired that nematic LCDs be developed which can efficiently work in information-system terminals. Nematic LCDs are available at present, but their operating abilities are sufficient for limited use only. When their screen size is increased, they no longer have an adequate viewing angle nor a sufficient response speed. The existing nematic LCDs inevitably find but limited uses.
Surface Stabilized Ferroelectric Liquid Crystal
In view of the above it has been proposed that smectic liquid crystal (more precisely, chiral smectic C-phase LC) be used. This is because smectic liquid crystal, whose molecules are more orderly aligned than those of nematic liquid crystal, exhibits a wide viewing angle and a high response speed. See N. A. Clark and S. T. Lagerwall, SSFLC: Surface Stabilized Ferroelectric Liquid Crystal, Appl. Phys. Lett. 36, 899 (1980). In an operating mode using SSFLC, SSFLC exhibits a response speed higher than that of nematic liquid crystal by about two to three orders of magnitude, and also a viewing angle comparable to that of a Cathode-Ray Tube.
In that operating mode, the helical structure of the chiral smectic C-phase LC is disintegrated by interaction of the liquid crystal and the orientation films, causing a spontaneous polarization. The polarization and the electric field interact with each other, generating a torque. The torque accomplishes switching of individual LC pixels. In this operating mode, SSFLC can be stabilized in only two states. SSFLC is stable when the direction of spontaneous polarization is along one of two opposite directions which are perpendicular to the interface between the orientation film and the liquid crystal. SSFLC can therefore have memory effect.
When SSFLC was announced, it was expected to be used without switching elements such as thin film transistors (TFTs). However, SSFLC is not used in practice. The reason is that a simple display mode using bistability cannot, in principle, display images in gray scale. However most LCDs for future use need to display full-color images, and the gray scale quality is indispensable for full-color images.
Nevertheless, to make use of the advantageous features of SSFLC, i.e., high response speed and wide viewing angle, researches has been conducted to develop methods of displaying gray-scale images by using SSFLC. Several methods have been proposed, some of which are disclosed in W. J. A. M. Hartmann, Ferroelectrics, 122, 1 (1991). In the methods disclosed in the thesis, the input signals are adjusted, enabling an SSFLC display to display gray-scale images. However, since SSFLC exhibits response characteristic equivalent to discontinuous switching operation called "domain inversion." Hence, it is impossible for the SSFLC to display gray-scale images without switching elements, only if the input signals are adjusted.
Antiferroelectric Liquid Crystal
Also proposed is an LCD using AFLC (Antiferroelectric Liquid Crystal) such as chiral smectic C.sub.A -phase (SC.sub.A *phase), instead of chiral smectic C-phase (SC*phase) which is a ferroelectric liquid crystal. (See A. D. L. Chandani, T. Hagiwara, T. Suzuki, Y. Ouchi, H. Takezoe, and A. Fukuda, Jpn. J. Appl. Phys. 27, L729 (1988).) In an operating mode using AFLC, AFLC is stabilized when the applied voltages is OV, in addition to the two stabilized states as SSFLC does. AFLC is characterized in that it assumes an antiferroelectic structure when no voltage is applied on it.
A representative relationship between the voltage applied on AFLC and the transmittance of AFLC is shown in FIG. 7A. In FIG. 7A, the voltage (V) is plotted on the abscissa, and the transmittance (T) is plotted on the ordinate.
As seen from FIG. 7A, the transmittance gradually increases as the voltage is lowered from 0 volt (changed toward negative direction). When the voltage is decreased to a particular negative value, the transmittance starts increasing sharply. Once the transmittance saturates (this value is regard as 100), it does not change even if the voltage is further lowered. As the voltage V is raised (changed toward positive direction) from this condition, the transmittance remains at 100 until the voltage increases to a particular value. Once the voltage reaches this value, the transmittance begins to decrease. Some time thereafter, the transmittance slowly decreases as the voltage is raised and reaches the minimum (this value regarded as 0) at the voltage of 0 V.
As the voltage is raised from 0 V, the transmittance again increases gradually as illustrated in FIG. 7A. When the voltage reaches a particular value, the voltage starts increasing sharply. Thereafter, the transmittance saturates, reaching 100. Then, the transmittance does not change even if the voltage is raised further. As the voltage V is lowered from this condition, the transmittance remains at 100 until the voltage reaches to a particular value. Once the voltage reaches this value, the transmittance begins to decrease sharply. Some time thereafter, the transmittance slowly decreases as the voltage is lowered and reaches the minimum of 0 at the voltage of 0 V.
As described with reference to FIG. 7A, the transmittance of AFLC has the minimum value (0) at 0 V, and the maximum value (100) at a specific negative value of voltage and a specific positive value of voltage. In other words, the transmittance of AFLC changes with the applied voltage over a certain ranges of both positive and negative voltage. This characterizing feature of AFLC can be applied to display gray-scale images.
It has been reported that an AFLC display can display gray-scale images without using active elements, when driven in a specific mode. For example, N. Yamamoto, N Koshoubu, K. Mori, K. Nakamura, and Y. Yamada, Ferroelectrics, 149, 295 (1993) report that a voltage ranging from V.sub.10 to V.sub.90 (FIG. 7A) may be applied to AFLC to display gray-scale images. The voltage range from V.sub.10 to V.sub.90 is used in both positive and negative voltage, this is because. This is because whether the voltage is positive or negative in this particular range, the transmittance of AFLC changes in the same manner. The transmittance varies sharply in the voltage range (V.sub.10 -V.sub.90), which means the range is very narrow. Although the voltage range is narrow, the voltage-transmittance curve is almost linear. AFLC can therefore display gray-scale images when applied with a voltage falling within this specific range. In addition, since voltage within this range can be used both positive and negative range, image sticking can be prevented easily. (Image sticking would occur if a voltage only positive or negative were applied to AFLC, maintaining AFLC in a fixed polarized state though the voltage is changing and causing AFLC to display an after-image.) Furthermore, AFLC has its optical axis rotating to a specific direction once after the application of voltage to it is stopped. This characteristic of AFLC works well to display gray-scale images.
However, the above-mentioned voltage range (V.sub.10 -V.sub.90) is too narrow to achieve gray-scale images of Satisfactory quality. Further, the voltage-transmittance curve varies with the temperature of AFLC. As shown in FIG. 7B, AFLC exhibits a voltage-transmittance curve I at a certain temperature and a different voltage-transmittance curve II at another temperature; namely, the voltage-transmittance relationship depends on the temperature of AFLC. The transmittance abruptly increases from the minimum value to the maximum value (or decreases from the maximum value to the minimum value) in specific voltage region. When a voltage within this region is applied to AFLC, the transmittance greatly changes with a slight temperature change.
Variation of the transmittance, caused by changes in temperature, makes no large problem so long as the AFLC display is operated in ON/OFF mode only, provided that the drive voltage is set on the basis of the temperature-dependency of the transmittance. To display gray-scale images, however, AFLC must be maintained in an extremely narrow temperature range. In view of this, the AFLC display cannot be used in practice to display gray-scale images.
Other Liquid Crystals
Further proposed are LCDs using chiral smectic C-type liquid crystal and operating in specific modes by using active elements. Examples of the operating modes of chiral smectic C-type LCDs are: DHF (Deformed Helix Ferroelectric Liquid Crystal) mode disclosed in J. Funfschilling and M. Schadt, J. Appl. Phys., 66, 15 (1989); and TFLC (Twisted Ferroelectric Liquid Crystal) mode disclosed in J. S. Patel, Appl. Phys. Lett., 60, 280 (1992).
The DHF display and the TFLC display need to have switching elements. Provided with switching elements, these liquid crystal displays are inevitably expensive. Nonetheless, they are advantageous in reliability of displaying gray-scale images. The DHF display and the TFLC display have a wider viewing angle than the TN-type LCD. But their viewing angles are smaller than those of the SSFLC display and the AFLC display. The DHF display has such a viewing angle, probably because DHF maintains its helical structure. The TFLC display has such a viewing angle, perhaps because TFLC has a twisted structure between the upper and lower orientation films.
Feasible Gray-Scale LCD
Of the above-mentioned liquid crystals which can serve to display gray-scale images, none have been found to have a response speed, a viewing angle, and temperature-dependence which are all satisfactory to provide a practical gray-scale LCD. Each of these liquid crystals has at least one problems and cannot be used to provide, in particular, a large-screen LCD. Demand for large-screen LCDs has been ever increasing. No other types of displays than LCDs can be thought of, as small, lightweight, low-power consumption displays. It is therefore desired that liquid crystals be developed which have a response speed, a viewing angle, and temperature-dependence all satisfactory to provide a practical gray-scale LCD.
As predicted, color LCDs will be dominant in the near future. To provide color LCDs which can display full-color images, liquid crystal having not only satisfactory response speed, viewing angle and temperature-dependence, but also an increased high contrast. For full-color images, the contrast directly determines the clearness of color, greatly influencing the quality of color images. Thus it is essential for the liquid crystal for use in a color LCD to exhibit high contrast.